docs: explain replay-to-realtime transition

[russlan23]

Closes #711.

What changed

Adds a focused "Replay Before Going Realtime" section to the execution-modes documentation. It explains how a past starttime and realtime=True produce a historical replay followed by a live handoff, and includes a Python 3.10-compatible example with an absolute future endtime.

The section also clarifies the roles of historical and realtime adapters and when to use csp.now() and csp.in_realtime().

Validation

• Parsed all three Python code blocks in Execution-Modes.md with ast.parse.
• Cross-checked timezone and end-time normalization against csp/impl/wiring/runtime.py.
• Cross-checked the replay-to-live behavior against the existing test_csp_in_realtime coverage in csp/tests/test_engine.py.
• Ran git diff --check.

This is a documentation-only change. The documentation lint workflow is awaiting the repository's first-time-contributor approval before it can run.